It is necessary to determine arteriovenous oxygen difference, or a-vO.sub.2 in order to determine cardiac output, which is an important physiologic parameter, during surgical procedures, post-operative monitoring and the management of critically ill patients. Cardiac output, or C.O. is equal to oxygen consumption, VO.sub.2, divided by arteriovenous oxygen difference, a-vO.sub.2. The relationship can be stated algebraically as: ##EQU1##
While there are non-invasive methods for determining oxygen consumption, VO.sub.2, at the present time, there are only invasive methods for determining arteriovenous oxygen
difference, a-vO.sub.2. The non-invasive method for determining arteriovenous oxygen difference of the present method would thus allow cardiac output to be determined without the need for invasive procedures with the attendant risks of infection, bleeding and other trauma. The present invention utilizes the optic disc as the monitoring site. This site was chosen because it has the most accessible peripheral arterioles and venules for non-invasive monitoring.
Prior art using the eye to measure oxygen saturation focused on different portions of the fundus. In U.S. Pat. No. 4,485,820, Flower disclosed a scleral contact lens with a fiberoptic apparatus which measured the hemoglobin saturation of the choroidal arteries. This corresponded to the arterial saturation in, primarily, premature infants. Rather than focusing on the optic disc, Flower utilized the capacity of the eye to serve as an integrating sphere, thereby providing the largest possible surface area to monitor choroidal (arterial) oxygen saturation. Also, Flower did not determine venous oxygen saturation and, therefore, did not determine arteriovenous oxygen difference.
In U.S. Pat. No. 4,877,322, Hill described the use of a collimated beam of light to view specific areas of the fundus, such as the macula, or the optic disc. The ratio of oxyhemoglobin to reduced hemoglobin of these particular areas allows the physician to detect macular degeneration or glaucoma at its early stages.
Novack, in U.S. Pat. No. 4,922,919, measures the oxidative metabolism in ocular tissue by taking advantage of the absorption peak of cytochrome c oxidase. Novack primarily employs an optical probe, which penetrates the ocular body. While Novack describes an alternative apparatus which consists of a contact lens, and subsequently mentions that the invention can also measure desaturated hemoglobin, arteriovenous oxygen difference is not determined. Additionally, Novack does not specifically target the optic disc.
In "The Choroidal Eye Oximeter: An Instrument for Measuring Oxygen Saturation of Choroidal Blood in vivo," IEEE Trans. Biomedical Engineering, Vol. BME-22, No. 3, pp. 183-193, 1975, Laing, R. A., et al., the authors make no distinction between the oxygen saturation of choroidal venous and arterial blood.
As mentioned above, arteriovenous oxygen difference, is a highly useful physiologic parameter. Shepherd et al. designed a spectrophotometer which could analyze the arteriovenous oxygen difference, when venous and arterial blood were pumped from dogs into cuvettes. See, Shepherd, A. P. and C. G. Burgar, "A solid-State Arteriovenous Oxygen Difference Analyzer for Flowing Whole Blood," Amer. J. Physiol., Vol. 232, pp. H437-H440, 1977. More recently, Suga et al. devised an instrument which measures not only arteriovenous oxygen content difference, but arterial and venous oxyhemoglobin saturation as well. See, Suga, Hiroyuki, et al., "Arteriovenous Oximeter for O.sub.2 Content Difference, O.sub.2 Saturation and Hemoglobin Content", Amer. J. Physiol. 257 (Heart Circ. Physiol. 26): pp. H1712-H1716, 1989. This is an invasive technique that requires cannulation of the subject's artery and vein.
In U.S. Pat. No. 4,305,398, Sawa discloses an eye oximeter for measuring the oxygen saturation of the blood in the fundus of the eye. Sawa recognized that the problem with this type of analysis is the difficulty in discriminating the reflection or absorption of light by the eye fundus blood from the reflection or absorption of light by the various cell layers in the eye fundus. Sawa's approach to solving this problem utilized the phenomenon in which visual pigments in photoreceptor cells may become transparent upon being illuminated by light.
In U.S. Pat. No. 3,565,529, Guyton, discloses a device for providing continuous measurement of the difference between the amount of oxygen in arterial and venous blood. Guyton disclosed an analyzer which determines the oxygen difference in arterial and venous blood. Light of a narrow wavelength is passed through venous blood and arterial blood that has been pumped into cuvettes. This required catheterization of an artery and a vein in order to pump blood through the cuvette and back into the subject's blood stream.
It is known that when red-free light (e.g. green) is shone into the optic fundus, the optic disc, which is free of photoreceptors, appears brighter than the surrounding fundus background. In fact, with the appropriate wavelength of green light, the retinal vasculature appears almost black. See, Paton, D., et al., In: Introduction to Ophthalmoscopy, Edited by B. A. Thomas, Kalamazoo, Mich.: Upjohn Company, 1979, p. 10. Additionally, the maximum absorption wavelength for mammalian rods is 500 nm, and for cones is 562 nm. See, Sharkov, A. V. and Yu A. Matveets, "Ultrafast Processes to Rhodopsins", In: Laser Picosecond Spectroscopy and Photochemistry of Biomolecules, 1987, p. 58.
It has been demonstrated that determination of oxyhemoglobin saturation values can be achieved in vessels as small as 12 microns diameter, using reflectance spectrophotometry. See, Fenton, B. M., et al., "Determination of Microvascular Hemoglobin saturations using Cryospectrophotometry," Amer. J. Physio. 259 (Heart Circ. Physiol. 28): pp. H1912-H1920, 1990. Also, Fenton states that the hematocrit for microvasculature tends to vary from one vessel to another. However, the arteriovenous oxygen difference is independent of hematocrit, at least up to a hematocrit value of 60 to 70%. See, Steinke, J. M., et al., "Role of Light Scattering in Spectrophotometric Measurements of Arteriovenous Oxygen Difference," IEEE TRANS. BIOMEDICAL ENGINEERING, Vol. BME-33, No. 8, pp. 729-734, 1986.
Accordingly, it is an object of the present invention to provide a method and apparatus which can detect arterial and venous oxyhemoglobin saturation in vivo, non-invasively, and atraumatically.
It is another object of the present invention to consistently determine and distinguish between venous and arterial oxyhemoglobin saturation by scanning the optic disc, thereby allowing a determination of the arteriovenous oxygen difference.
It is yet a further object of the present invention to provide a method and apparatus for scanning the optic disc region of the fundus in a linear or curvilinear pattern, thus allowing more than one venule and arteriole pair to be analyzed.
Still another object of the present invention is to provide an apparatus with the capability of sensing the optic disc region of the fundus and accordingly monitoring the optic disc when the optic disc is targeted by the scanning light sources, thus reducing the inaccuracy that results from head and eye movement and that also results from absorbance and reflectance of light by the surrounding fundus tissue layers.
Still yet another object of the present invention is to provide a method and apparatus for determining the arteriovenous oxygen difference in a given patient over a period of time and series of readings thus allowing the patient in question to provide his or her own base line reference.
It is yet another object of the present invention to provide an apparatus for non-invasively determining arteriovenous oxygen difference that is sufficiently mobile to allow examination of multiple patients in separate rooms or facilities.
Other objects and advantages over the prior art will become apparent to those skilled in the art upon reading the detailed description together with the drawings as described as follows.